This invention concerns the improvement of the finish of electro-deposited tinplate. Specifically, the invention relates to compounds which may be used as a flux on the tinplate prior to the reflow process and ensure the production of a bright, reflective tin coating free from surface defects.
The electro-deposition of a thin coat of tin onto steel strip to impart corrosion resistance and decoration is described in detail in the book xe2x80x9cThe Technology of Tinplatexe2x80x9d by Hoare et al., published by St. Martins Press, New York, 1965. Coiled steel strip is unwound and passed through cleaning and acid pickling stations before entering the tinplating bath where the steel strip acts as the cathode and collects a surface coat of tin with a characteristic smooth matte finish. In the subsequent process, known as flow-brightening, flow-melting or reflow, the tinplate is heated to above the melting point of the tin by the passage of an alternating current, for example, and immediately quenched. The advantages of this operation are two-fold. Firstly an iron-tin alloy layer is formed between the steel base and tin coat thereby improving corrosion resistance and secondly the matte deposit is converted to a pleasing bright reflective surface.
A serious defect which may be generated through reflow is that of xe2x80x9cwoodgrainxe2x80x9d where the tinplate surface takes on the non-uniform appearance of longitudinally cut wood. Woodgrain is thought to be caused by localised de-wetting in areas where slow cooling allows solidification prior to quenching, see xe2x80x9cThe Origins of Woodgrain and Related Phenomenon on Tinplatexe2x80x9d, P. G. Harris and I. M. Notter, ITRI Publ. (1992), 727 (Fifth International Tinplate Conference, 1992), 56-65. If an alternating current is used to heat the plate the solid/liquid front can be periodically stationary allowing de-wetting and the production of a banded structure. Thin areas in the steel base cause localised hot spots, distorting the bands into the woodgrain swirls.
The de-wetting process itself can be influenced by tin oxide formation during reflow as described in xe2x80x9cWoodgrain on Tinplatexe2x80x94How it Develops and How it can be Avoidedxe2x80x9d, W. Pappert and V. Tenhaven, ITRI Publ. (1976), (First International Tinplate Conference, 1976), 83-9. These authors recommend xe2x80x9cfluxingxe2x80x9d the tinplate with citric acid (an antioxidant) prior to reflow to inhibit woodgrain formation. The term flux in this context refers to a substance which aids, induces or otherwise actively participates in fusing or flowing.
Other fluxes used to inhibit woodgrain include phenolsulphonic acid (Belousova et al, Stal (USSR), May 1991), naphtholdisulphonic acids (U.S. Pat. No. 5,427,677), gluconic, glutamic and tartaric acids (JP 58001085) and citrazinic, chelidamic acid, chelidonic acid and cyclohexene-1,2-dicarboxylimide (JP 60063395).
The prior art fluxes typically have low thermal stabilities and must be used in high concentrations (5 g/l and above).
The requirements of a flux are high reduction potential, high affinity for the metal and high thermal stability. In commercial operation, it is desirable that the flux retains its activity in the presence of dilute electrolyte which contains tin ions, other sulphonic acids, such as phenolsulphonic acid or toluenesulphonic acid and the like and optionally antioxidents. This is because electrolyte may be dragged from the plating bath into the fluxing bath by the moving steel strip or because it is operationally convenient to dissolve the fluxing agent in dilute electrolyte to form the fluxing bath. It is thus advantageous that fluxing agents of the present invention are compatible with different commercial electrolytes.
This invention discloses a new class of flux materials to be applied to matte tinplate prior to reflow where the flux helps achieve a uniform bright tin finish. The flux Materials are generally classified as dihydroxy or polyhydroxy phenyl compounds containing one or more sulphonic acid or sulphonate groups. They prevent the formation of surface defects and are effective at lower concentrations than prior art compounds. It has been discovered that by combining the high education potential of dihydroxybenzenes with the solubility and thermal stability imparted by sulphonic acid or sulphonate groups a class of fluxing agents is produced which strongly inhibits woodgrain formation during reflow.
Thus, the present invention includes a composition for use in a tinplating process, a method of treating tinplate prior to reflow and a method for producing bright tinplate by immersing matte tinplate into an aqueous solution of the fluxing compound, removing and drying the matte tinplate in order to generate a tinplate coated with the fluxing agent and heating the coated matte tinplate to a temperature above the melting point of the tin, but below that of the steel, and quenching to generate a bright tinplate free from woodgrain.
According to the present invention there is provided a composition for use as a flux material in a tin plating process, which comprises an aqueous solution of a hydroxy phenyl compound of the general formula: 
wherein:
M=A cationic species, preferably H, substituted or unsubstituted ammonium, alkali metal, alkaline earth metal or tin.
R=H, C1-C6 linear or branched alkyl, alkoxyl or alkenyl, or aryl (which may may be substituted)
m=2 or 3
n=1 or 2,
the composition contains from about 0.1 g/l to saturation of said compound of general formula I.
The compound of formula I may be in solution in an aqueous electrolyte.
Also according to the present invention there is provided a process for treating matte tinplate prior to reflow which comprises contacting the tinplate with a composition of the invention.
The preferred method of contacting the tinplate with the composition of the invention is by immersing the tinplate in a bath of the composition.
Further, according to the present invention there is provided a process of tin plating which comprises the steps of electrolytically plating steel strip with a matte finish of tin, treating the matte tin with a composition in accordance with the present invention, drying the matte tinplate, heating the tinplate to above the melting point of tin to reflow the matte tin coating and quenching to produce a bright tin deposit. The flux is applied to the matte tinplate as an aqueous solution, the concentration of the fluxing compound in the solution being from about 0.1 g/l to saturation and preferably from about 0.6 g/l to 10 g/l, and most preferably from 1 g/l to 5 g/l.
The flux materials used in the present invention are thus about five times more efficient than naphthalenesulphonic acid compounds (recommended dose of greater than 5 g/l) and about ten times more efficient than phenolsulphonic acid (typically dosed at 10 g/l), significantly reducing costs and environmental impact. The compatibility of the novel fluxes with different electrolyte systems allows the use with different plating technologies.
Preferred Examples of fluxing compounds are:
1,2-dihydroxybenzene-4-sulphonic acid;
1,2-dihydroxybenzene-3,5-disulphonic acid;
1,2-dihydroxy-3-methylbenzene-4 or 5-sulphonic acid
1,2-dihydroxy-3-ethoxybenzene-4 or 5-sulphonic acid
1,2-dihydroxy-4-propylbenzene-5-sulphonic acid
1,2-dihydroxy-4-(2-sulphoethyl) benzene
1,3-dihydroxybenzene-4-sulphonic acid
1,3-dihydroxybenzene-4,6-disulphonic acid
1,4-dihydroxybenzene-2-sulphonic acid
1,4-dihydroxybenzene-2,5-disulphonic acid
1,3,5-trihydroxybenzene-2-sulphonic acid
1,3,5-trihydroxybenzene-2,4-disulphonic acid
1,2,3-trihydroxybenzene-4-sulphonic acid
1,2,3-trihydroxybenzene-4,6-disulphonic acid
1,2,4-trihydroxybenzene-5-sulphonic acid
1,2,4-trihydroxybenzene-3,5-disulphonic acid
Especially preferred fluxing compounds are:
1,2-dihydroxybenzene-4-sulphonic acid;
1,2-dihydroxybenzene-3,5-disulphonic acid;
1,4-dihydroxybenzene-2-sulphonic acid
1,4-dihydroxybenzene-2,5-disulphonic acid
The fluxing compounds may be conveniently prepared by direct sulphonation of the corresponding hydroxybenzene with oleum or sulphuric acid. The corresponding salts may be prepared by neutralisation of the sulphonation mixture with alkali, for example sodium hydroxide (M=Na); potassium hydroxide (M=K) calcium hydroxide (M=Ca); ammonia or an organic base (M=NH4 or substituted NH4). Alternatively some of the above compounds are available from commercial sources (see below). Fluxing compounds containing a sulphonic acid group substituted in the side chain R may be prepared by reaction of the corresponding halide with sodium sulphite and those containing a sulphato group by treatment of the corresponding alcohol with sulphuric or sulphamic acid.
In a preferred embodiment of the invention the fluxing compounds are prepared by direct sulphonation of the corresponding hydroxybenzene with sulphuric acid or oleum and the acid solution is added directly to the fluxing bath to the required concentration.
The concentration in the compositions of this invention with respect of the pure substance is 0.1 g/l to saturation and preferably 0.6 g/l to 10 g/l and most preferably from 1 g/l to 5 g/l. The compositions of the invention may contain one or more compounds of formula I and the composition may contain other ingredients, eg. dilute electrolyte.